"Every movie is going to be a different villain," Whedon told Entertainment Weekly. "Loki's awesome, and he is awesome in Thor 2. But The Avengers is a different thing.

"I worked with Tom more than any other actor [on The Avengers], because he was the only villain and there were six heroes. And he was as great as anyone I've ever worked with, and I get why people loved the character. But it's an Avengers movie."

Whedon was also asked about his tongue-in-cheek promise that the sequel would feature "death, death, death", suggesting that he would consider killing one of the Avengers if the story demanded it.

"I'd have to have a really good reason, a really great sequence for [Marvel executives] to go, 'We'll cut off a potential franchise, that's fine!' They know, as any good studio does, that without some stakes, some real danger, how involved can we get?

"We don't just rule it out across the board, but neither is the mission statement 'who can we kill?' We try to build the story organically and go, 'How hard can we make it on these people?'"

The Avengers: Age of Ultron will begin filming in London in early 2014, with a release date set for May 1, 2015.